Read a thermocouple (RS485 to Usb)

[Gareth R]

I have recently acquired a thermocouple devise so that i can measure the temperature of a heating element in a vacuum chamber. I was wondering if people could advise me on which would be the best route to read the temperature from the thermocouple. It is RS485 connection and my supervisor and i were thinking about buying an RS485 to USB converter to connect it to the PC and then try to write a labview program to read it. Is this the best option or is there a better method?

With regards to communicating with labview, would i have to make a low level read and write program or is there a better way?

Thank you for any help

Gareth R

[Dennis_Knutson]

A USB->485 converter should work just fine. There are also pci cards that do the same thing and converters that plug into an existing RS-232 connector. To communicate in LabVIEW, you would use the serial VISA functions. There are shipping examples that can help you get started. The Instrument I/O Assistant can also be used. Before you start writing any code, I would recomend you test communciations with a terminal emulator program like Hyperterminal. Once you've got that working, you know that the com settings and cable are correct and can concentrate on the LabVIEW program.

[SachaE]

Hi,

  just a quick note on the USB-RS485 converter :

RS485 networks require bias and termination (for more information check out here : http://digital.ni.com/public.nsf/websearch/12BB6CD900B2D0EF862564980050F5A7?OpenDocument 
and here
http://www.ni.com/support/serial/resinfo.htm
)

From the Serial Hardware and Software Help that you get when you've installed NI Serial (free download here :
http://digital.ni.com/softlib.nsf/websearch/9400D483DF9DB3BC8625705C005F7C6B?opendocument&Submitted&...
)

On the USB-485 hardware, there are programmatically controlled onboard bias resistors. In addition, the USB-485 two and four-port hardware has user-configurable socketed bias resistors. By default, the USB-485 hardware uses the programmatically controlled bias resistors, which are connected to the receive signals of each port to maintain a known state when the bus is idle. The connections are made as follows:

• RXD+ and CTS– are pulled up to +3.3 V
• RXD– and CTS+ are pulled down to GND

So if you need to add your own bias resistors in, then you need to specifically disable the programatically controlled ones.

Now on the USB 485 1 port, it's a moulded plug, so you can't physically get to the internals to put a resistor in, so you have to do it on the 9 way D-type

pins 1 (GND)  to 5 (RXD-) with a 620Ohm Bias resistor
pins 4 (RXD+) to 7 (RTS-) with a 620Ohm Bias resistor
pins 4 (RXD+) to 5 (RXD-) with a 120Ohm Termination resistor
pins 9 (TXD+) to 8 (TXD-) with a 120Ohm Termination resistor

but as I said, that will depend on your actual network and where (and whether) you need bias resistors since they only go on one node on the RS 485 network.

Hope that helps

Thanks

Sacha Emery
National Instruments (UK)